1. Field of the Invention
The present invention relates to multilayer baluns, and in particular, to a multilayer balun for use as a balanced-to-unbalanced transformer or a phase transformer of ICs for radio communication equipment, a hybrid integrated circuit module having the same, and a multilayer substrate.
2. Description of the Related Technology
The Marchand balun shown in FIG. 15 is generally used as a balanced-to-unbalanced signal circuit for high frequency signals. The Marchand balun 10 is a distributed constant circuit with a simple structure. That is, the Marchand balun 10 includes two quarter-wave transmission lines 11 and 12 which are connected in series. An open end of the quarter-wave transmission line 11 is connected to an unbalanced terminal 15. The two quarter-wave transmission lines 11 and 12 constitute a half-wave transmission line. Furthermore, a quarter-wave transmission line 13 is opposed to the quarter-wave transmission line 11, and a quarter-wave transmission line 14 is opposed to the quarter-wave transmission line 12. The end of the quarter-wave transmission line 13 adjacent to the unbalanced terminal 15 of the quarter-wave transmission line 11 is grounded, and the other end is connected to a balanced terminal 16. The end of the quarter-wave transmission line 14 adjacent to the connection of the quarter-wave transmission line 11 and the quarter-wave transmission line 12 is connected to a balanced terminal 17, and the other end is grounded.
Since the Marchand balun 10 has a simple structure as described above, it has the advantage of providing preferable characteristics in a high frequency band even with a small size. In view of this technical background and because of recent widespread use of radio equipment, parts manufacturers are working on the development of multilayer Marchand baluns.
Known examples of this type of multilayer baluns are a balun transformer disclosed in Japanese Patent No. 2773617 and a, multilayer balun transformer disclosed in Japanese Patent No. 2990652.
As shown in FIG. 16, a balun transformer 20 disclosed in Japanese Patent No. 2773617 has dielectric substrates 21 to 25 stacked in that order, on each of which an earth electrode or a stripline is formed. That is, the uppermost first dielectric substrate 21 has an earth electrode 31 which is connected to a grounding external terminal electrode (not shown). The upper surface of the second dielectric substrate 22 has a straight stripline 32, whose first end 32a is connected to an unbalanced external terminal (not shown). The upper surface of the third dielectric substrate 23 has spiral striplines 33 and 34 constituting the above-mentioned half-wave transmission line. The striplines 33 and 34 are substantially symmetrical about the center of the dielectric substrate 23. A first end 33a of the stripline 33 at the center of the spiral is connected to a second end 32b of the stripline 32 through a via conductor (not shown). A second end 33b of the stripline 33 is connected to a first end 34a of the stripline 34 located outside the spiral, while a second end 34b of the stripline 34 at the center of the spiral is open.
The upper surface of the fourth dielectric substrate 24 has two spiral striplines 35 and 36 that constitute the above-mentioned two quarter-wave transmission lines, the striplines 35 and 36 being opposed to the striplines 33 and 34, respectively. A first end 35a of the stripline 35 opposed to the stripline 33 is connected to a first balanced external terminal (not shown). A second end 35b of the stripline 35 is connected to an earth electrode 37 formed on the upper surface of the fifth dielectric substrate 25 through a via conductor (not shown). A first end 36a of the stripline 36 opposed to the stripline 34 is connected to a second balanced external terminal (not shown). A second end 36b of the stripline 36 is connected to the earth electrode 37 on the fifth dielectric substrate 25 through a via conductor (not shown). The earth electrode 37 is connected to a grounding external terminal electrode (not shown).
Thus, the Marchand balun (balun transformer) shown in FIG. 15 is formed.
As shown in FIG. 17, a multilayer balun transformer 40 disclosed in Japanese Patent No. 2990652 has spiral striplines on different layers so that the mounting area for the balun transformer 20 is reduced.
Specifically, the multilayer balun transformer 40 is formed of dielectric substrates 41 to 48 layered in that order, on each of which an earth electrode or a stripline is formed. That is, the uppermost first dielectric substrate 41 has an earth electrode 51 which is connected to a grounding external terminal electrode (not shown). The upper surface of the second dielectric substrate 42 has a substantially straight stripline 52, whose first end 52a is connected to an unbalanced outside terminal (not shown). The upper surface of the third dielectric substrate 43 has a spiral stripline 53 constituting a half of the above-mentioned half-wave transmission line. A first end 53a of the stripline 53 at the center of the spiral is connected to a second end 52b of the stripline 52 through a via conductor (not shown). A second end 53b of the stripline 53 is connected to a first end 57a of a stripline 57 outside the spiral, to be described later.
The upper surface of the fourth dielectric substrate 44 has a spiral stripline 54 that constitutes one of the above-mentioned quarter-wave transmission lines in such a manner that the stripline 54 is opposed to the stripline 53. A first end 54a of the stripline 54 outside the spiral is connected to one balanced external terminal (not shown). A second end 54b of the stripline 54 at the center of the spiral is connected to an earth electrode 55 formed on the fifth dielectric substrate 45 through a via conductor (not shown).
The upper surface of the fifth dielectric substrate 45 has the earth electrode 55 which is connected to a grounding external terminal electrode (not shown).
The upper surface of the sixth dielectric substrate 46 has a spiral stripline 56 that constitutes the other of the above-mentioned quarter-wave transmission line in such a manner that the stripline 56 is opposed to the stripline 57. A first end 56a of the stripline 56 outside the spiral is connected to the other balanced external terminal (not shown). A second end 56b of the stripline 56 at the center of the spiral is connected to the earth electrode 55 formed on the fifth dielectric substrate 45 through a via conductor (not shown).
The upper surface of the seventh dielectric substrate 47 has a spiral stripline 57 that constitutes a half of the above-mentioned half-wave transmission lines. A first end 57a of the stripline 57 outside the spiral is connected to the second end 53b of the stripline 53 through an external electrode (not shown). A second end 57b of the stripline 57 is open.
The upper surface of the eighth dielectric substrate 48 has an earth electrode 58 which is connected to a grounding external terminal electrode (not shown).
Thus the Marchand balun (balun transformer) shown in FIG. 15 is formed.
Japanese Patent No. 2990652 further discloses a multilayer balun transformer 40B shown in FIG. 18, in which two quarter-wave striplines 53, 57 that constitute a half-wave transmission line are connected by via conductors in place of the external electrode.
The balun transformer disclosed in Japanese Patent No. 2773617 has a conventional structure in which a Marchand balun is of a multilayer type. This has the problem of difficulty in reducing the size because it needs an area for the half-wave stripline. Although this balun transformer can be reduced in height, the interval between the stripline and each earth electrode is reduced. This increases the coupling capacity between them, posing the problem of difficulty in impedance matching.
The multilayer balun transformer disclosed in Japanese Patent No. 2990652 has two-layered half-wave transmission lines, so that it has an advantage in that the balun can be formed in an area half or smaller than the balun transformer disclosed in Japanese Patent No. 2773617. This further has an advantage in that the impedance of the quarter-wave striplines on the balanced input and output sides can be adjusted independently. However, this balun transformer has a ground plane between the two-layered half-wave transmission lines to prevent deterioration of the characteristics due to electromagnetic coupling. Therefore, it has the problem of difficulty in reducing the height.